The field of the invention is processes for liquid purification or separation. The invention is particularly concerned with an improvement in a process for filtering off suspended particles from liquids, especially water, by using inorganic artificially produced filter sand.
The state of the art for producing the raw materials of the present invention may be ascertained by reference to U.S. Pat. Nos. 3,184,371 and 3,261,894, of Alois Seidl, U.S. Pat. No. 3,331,694 of Heidrich et al, and Application Ser. No. 56,075.
According to U.S. Pat. Nos. 3,184,371 and 3,261,894, a preliminary product is produced by:
(A) FORMING A SOLUTION CONTAINING IN PARTS BY WEIGHT ABOUT 1 PART ALKALI METAL SILICATE, WHEREIN THE MOLAR RATIO OF Na.sub.2 O:SiO.sub.2 is 1:2.5 to 1:4.5, 0.02 to 0.8 parts of inorganic fibrous silicate or finely powdered silicate and 2-10 parts of water;
(B) ADDING MEANS FOR PRODUCING FOAMING GAS TO THE SOLUTION; AND
(C) HEATING THE SOLUTION TO ABOUT 140.degree.-360.degree. C to drive off any free water phase.
According to U.S. Pat. No. 3,331,694 a preliminary product is produced by ball milling a typical window glass composition to a degree of fineness that 80 percent passes a 50 .mu. sieve and then adding 0.3 to 1.5 percent of a car bonaceous foaming agent, such as carbon-black or lamp-black to the ball mill.
According to Application Ser. No. 56,075, a method of producing fillers for bricks, chamotte, tiles and the like structural materials is disclosed comprising the steps of:
(A) DISSOLVING ABOUT 0.02 TO 0.7 PARTS BY WEIGHT OF FIBROUS INORGANIC SILICATE IN ONE PART BY WEIGHT OF AN AQUEOUS ALKALI METAL SILICATE SOLUTION;
(B) EVAPORATING THE MIXTURE TO A WATER CONTENT OF ABOUT 5-20 PERCENT BY HEATING;
(C) GRANULATING THE DRIED MATERIAL TO A PARTICLE SIZE BETWEEN ABOUT 0.5-20 MM;
(D) COATING THE GRANULATED MATERIAL WITH A DISPERSION OF A HIGH MELTING INORGANIC COMPOUND HAVING A MELTING POINT BETWEEN ABOUT 1,200.degree. TO 2,500.degree. C; and
(E) HEATING THE COATED MATERIAL BETWEEN ABOUT 600.degree. AND 1000.degree. C to produce foamed glass particles having a melting point between about 500.degree. to 900.degree. C.
In the purification of surface and ground water to obtain useful water, water for industrial use, and drinking water, the removal of mechanically suspended impurities by filtration is a conventional process stage in the purification, especially in the case where a flocculation with metallic salts takes place in the course of the water purification process.
Most simply, the filtration is conducted by means of sand and gravel layers, wherein large area filter tanks are provided with layers of coarse gravel, fine gravel, and sand so that the grains become finer from the bottom toward the top. Apart from the fact that the filter efficiency per square meter of filtering surface is low, due to the low filtration rate, the lifetime of the filters is short, because practically only the surface of the filter is effective, and a filtration in the lower strata does not occur. The filtration rate can be increased considerably by employing closed pressure filters in place of the simple gravity filters. Since also in this instance only the surface of the filter medium becomes effective, the filter service lifetime is likewise short. This lifetime can be considerably lengthened by directing filtration against the force of gravity, i.e., from the bottom toward the top, or from the coarse grains to the fine grains, because in this case the precipitate is retained in the deeper strata of the filter mass. However, in this mode of operation, the filtration rate must be lowered to a few meters per hour, in order to avoid turbulence in the layer of fine grains disposed on the pure water side -- and thus to avoid secondary turbidity.
The state of the prior art methods for filtering against the force of gravity (from the coarse grains to the fine grains) may be ascertained by reference to U.S. Pat. Nos. 3,171,801 to Rice et al, which issued Mar. 2, 1965, and 3,202,286 of Smit, which issued Aug. 24, 1965. In the patent of Rice et al, as shown in the drawing, water to be purified is passed downward through a series of filters wherein the filters have coarse to fine particles. As shown in FIGS. 1 and 2 of Smith, the flow through the filter from the coarse to fine particles is accomplished against gravity by the use of a vacuum.
Optimum filtration conditions would prevail if the filter layer were constructed so that the bottom layer exhibited a granular size of about 0.2-1 mm, and thereabove the layers were of a grain size of 1-1.5 mm., then of 1.5-2 mm., and finally in the top layer of a grain size of about 2-4 mm. Such a filter would be operable at high filtration rates in the direction of gravity, would have a filter effect throughout the entire thickness of the filter bed, and thus would have a very long lifetime at a very low pressure loss. The breakthrough safety would be high, since the extremely fine-grained filter layer at the water exit has the effect of a safety zone at any load condition. A filter constructed in this way has not existed in practice heretofore, however, because it could not be regenerated by backwashing. For during the sedimentation after any washing, the coarse particles would be deposited first of all, and the fine particles last of all; consequently, the layering sequence would be reversed and all advantages initially obtained would again be eliminated.
Attempts to obtain the sedimentation sequence in the desired way by the selection and utilization of products, the granular density of which is lower at a larger diameter than in the case of smaller diameter only enjoyed partial success. The reason for this is that the grain density of the substances usable for the filtration could be varied maximally only between 1.8 and 2.5 kilograms/liter. Only the combination of hydroanthracite/quartz sand has found acceptance in practice, but the high price of hydroanthracite has prevented a wide use thereof.